In a spark ignition-type internal combustion engine, generally, the lower the engine load is, the worse the heat efficiency becomes. Therefore, it has been proposed that the mechanical compression ratio in the low engine load side operation area is increased by a variable compression ratio mechanism higher than that in the high engine load side operation area, to increase the expansion ratio and thereby improve the heat efficiency (refer to Japanese Unexamined Patent Publication Nos. 2007-303423, 2005-067591, 2006-283601, and 2002-038961).
In such a spark ignition-type internal combustion engine with a variable compression ratio mechanism, even when in the low engine load side operation area the engine load is a set engine load or over, it is preferable to increase the engine output by supercharging. For this purpose, when the engine load reaches the set engine load, for example, the electromagnetic clutch connects the engine drive shaft and the supercharger to make the supercharger in a non-operating state operate and thus the boost pressure is increased like a step change.